Question

I have few questions about this code:

<?php 
   $iv_size = mcrypt_get_iv_size(MCRYPT_RIJNDAEL_256, MCRYPT_MODE_ECB); 
   $iv = mcrypt_create_iv($iv_size, MCRYPT_RAND); 
   $key = "This is a very secret key"; 
   $text = file_get_contents('path/to/your/file'); 
   echo strlen($text) . "\n"; 

   $crypttext = mcrypt_encrypt(MCRYPT_RIJNDAEL_256, $key, $text, MCRYPT_MODE_ECB, $iv); 
   file_put_contents('path/to/your/file', $crypttext);    
?> 

It encrypts the file just fine, however it adds additional nulls at the end, so if I encrypt:

 a test string is this one
    and here is a new line 

once decrypted becomes:

a test string is this one
    and here is a new line 000000000000000

What’s going on?

Second, is MCRYPT_RIJNDAEL_256 compatible with AES-128?

Finally, how would I let another party decrypt a file I’ve encrypted? They would need to know which encryption was used and I am not sure what to tell them.

Answer 1

MCRYPT_RIJNDAEL_128 is AES-128, MCRYPT_RIJNDAEL_256 is AES-256 – just another name:

[…]The standard comprises three block
ciphers, AES-128, AES-192 and AES-256,
adopted from a larger collection
originally published as Rijndael.originally published as Rijndael.[…]

[…]The Rijndael cipher was developed by
two Belgian cryptographers, Joan
Daemen and Vincent Rijmen, and
submitted by them to the AES selection
process. Rijndael (pronounced “Rhine
dall”) is a wordplay with the names of
the two inventors.[…]

The \x00 characters you encounter at the end of the decrypted string are the padding required for some block ciphers (with ECB being such a block cipher). Mcyrpt uses NULL-padding internally if the input data needs to be padded to the required block length. There are other padding modes available (which have to be user-coded when using Mcyrpt), namely PKCS7, ANSI X.923 or ISO 10126. NULL-padding is problematic when encrypting binary data that may end with one or more \x00 characters because you can’t detect where the data ends and the padding starts – the other padding modes mentioned solve this kind of problem. If you’re encrypting character data (strings) you can easily trim off the trailing \x00 by using $data = trim($data, "\x00");.

To decrypt the data you sent to a consumer, the consumer would need to know the IV (initialization vector) ($iv), the algorithm used (MCRYPT_RIJNDAEL_256/AES-256), the encryption mode (ECB), the secret encryption key ($key) and the padding mode used (NULL-padding). The IV can be transmitted with the encrypted data as it does not need to be kept secret:

The IV must be known to the recipient
of the encrypted information to be
able to decrypt it. This can be
ensured in a number of ways: by
transmitting the IV along with the
ciphertext, by agreeing on it
beforehand during the key exchange or
the handshake, by calculating it
(usually incrementally), or by
measuring such parameters as current
time (used in hardware authentication
tokens such as RSA SecurID, VASCO
Digipass, etc.), IDs such as sender’s
and/or recipient’s address or ID, file
ID, the packet, sector or cluster
number, etc. A number of variables can
be combined or hashed together,
depending on the protocol.depending on the protocol.